1. Field of the Invention
This invention relates to a novel stripping solution and its use for removing positive photoresist from the surface of an inorganic substrate.
2. Brief Description of the Prior Art
Where a pattern is to be formed on an inorganic substrate, such as metallic circuitry in the manufacture of printed circuit boards and integrated circuits, it can either be put down selectively in the appropriate areas such as by electroplating, or put down over the entire substrate and selectively removed by chemical etching. A method well known in the art of effecting such selective action involves forming a layer of resist over the substrate (or metal-coated substrate, hereafter referred to collectively with non-metal-coated substrate as "substrate"), in those areas to be shielded from metal deposition (or metal removal). Typically, the resist layer can be formed of a polymeric organic substance which is substantially unaffected by the metal deposition (or metal removal) process, and thus is able to protect the underlying areas of the substrate. For this purpose, various photoresists are known, that is, substances which form the desired masking pattern on the substrate upon exposure to activating light through a photographic image.
Photoresists are generally of two types, positive photoresists and negative photoresists. Prior to exposure to activating light, x-ray or electron beam radiation, a positive resist is insoluble in a developer solution. Upon exposure, the light-struck portion of the photoresist becomes soluble in the developer while the unexposed areas remain insoluble. Upon development, usually in aqueous basic solution, the solubilized portions are removed, leaving the insoluble portions of the resist layer on the substrate in the pattern of the photographic image.
Negative acting photoresists are chemically distinct from positive resists, and become insolubilized upon exposure to activating light. Negative photoresists are, for example, cyclicized rubber-based resists, while positive photoresists typically comprise, for example, a novolak, resin rendered insoluble in base solution by incorporating light-sensitive material. Upon exposure to actinic radiation, the light-sensitive material is chemically altered, and no longer protects the resist from dissolving in the developer. Other types of both positive and negative photoresists are also known.
Subsequent to putting down (or etching away) metal or oxide layers on the exposed portions of the substrate, the resist layer must be removed. The removal process must not adversely affect either the substrate or the metal layer. The substrate can be silicon dioxide coated silicon wafer, as in the manufacture of semiconductor microcircuitry and the metallic microcircuitry is typically aluminum or aluminum alloy. Other materials used in I.C. manufacture, or used for example in the manufacture of printed circuit boards, include a wide variety of inorganic substrate materials such as garnet crystal, L.E.D. and L.C.D. surfaces; e.g., indium oxide, iron oxide, chrome and virtually every other metal. The means employed for stripping the resist must act without corroding, dissolving, or dulling the surface of such materials, and without chemically altering the material forming the substrate.
Prior art methods for removing positive photoresists include the use of organic stripping compositions comprising phenol and linear alkyl benzene sulfonic acids, as exemplified in U.S. Pat. No. 3,871,929. Other known organic stripping compositions contain organic sulfonic acids of the formula R--SO.sub.3 H, wherein R is an organic radical, organic solvent, fluoride ion and optionally, phenol, as disclosed in U.S. Pat. No. 4,165,295.
Prior art inorganic stripping compositions include aqueous sulfuric acid compositions containing fluoride ion, as exemplified in U.S. Pat. No. 3,932,130, U.S. Pat. No. 3,654,001 and U.S. Pat. No. 3,080,071.
The above-mentioned inorganic stripping compositions are disadvantaged by their tendency to cause undesirable etching of silicon dioxide substrates, which effect is likely caused by their fluoride ion content. The above-mentioned organic stripping compositions have the disadvantage of producing undesirable dulling of the surface of aluminum circuitry put down on the substrate, or of causing undesirable etching of silicon dioxide substrate. Stripping compositions containing phenolics are significantly disadvantageous in that phenolics are toxic, poorly waste-treatable, and can actually lower stripping efficiency when employed in compositions for stripping positive-type photoresists. The use of phenol-type strippers is rapidly becoming untenable due to environmental considerations. [L. H. Kaplan and B. K. Bergin, J. Electrochemical Soc., 127, 386 (1980).]
Many prior known stripping compositions are disadvantaged in being insufficiently effective against certain resists. In particular, known stripping compositions are insufficiently effective against those resists which, during the course of device manufacture, have been subjected to elevated temperatures, such as during exceptionally high temperature post-bake, to high-energy processing, such as during ion implantation steps, or to exceptionally harsh chemical environments, such as platinum and mesa etch. This decreases their usefulness generally, and restricts the flexibility of processing lines in which they are used.
Certain known stripping solutions for positive photoresists are opaque or have coloration which prevents their use in stripping processes employing automatic end point determination.
It is an object of the present invention to provide a stripping composition which efficiently removes positive-type photoresist. In this regard, it is a further object to provide a stripping solution which removes the resist layer but does not significantly adversely affect the metal layer or the inorganic substrate including, for example, substrates of silicon, silicon dioxide (thermal or sputter grown), and polysilicon, and aluminum and aluminum alloy.
It is another object of the present invention to provide a stripping composition which readily removes ion-implanted positive photoresist films.
It is another object of the present invention to provide a stripping composition which is optically clear and free from coloration and which therefor lends itself to automatic end point detection.
It is another object of the present invention to provide a stripping composition which does not contain either fluoride ion, or phenolic compounds or other substances difficult to waste-treat.
It is another object of this invention to provide a stripping composition which is a liquid at room temperature.
It is another object of the present invention to provide a method for stripping positive-type photoresists from inorganic substrates, including metalized substrates, particularly in the manufacture of integrated circuitry and printed circuit boards.